This is not a duplicate, the other question asks about adopting the Babylonian calendar system, please remove the notice.
The original names for Hebrew months, as they appear in Torah - are just serial numbers, counting from Nissan, being the first month, Iyar the second etc. As in:
"ויסעו מרעמסס בחודש הראשון, בחמישה עשר יום לחודש הראשון" (Num 33,3)
"בשנת הארבעים, לצאת בני-ישראל מארץ מצריים, בחודש החמישי, באחד לחודש" (ibid 28)
Many interpreters mention that this is one of the ways of a remembering the Exodus.
This is consistent with the ruling of the Gemmorah that "ישראל מונים ללבנה", which literally means "Israel count months". However, the current names we use were brought from the Babylon exile, and those names are proper names, lacking the serial counting, and, therefore, remembering Nissan and the Exodus.
How come such a big Deoraisoh issue was forgotten, and the Sages decided to override the naming of the Torah and the remembrance of the Exodus in counting Hebrew months?